The present invention relates to communications systems and methods, and more particularly, to spread spectrum communications systems and methods.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data), as described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996). Proposed next-generation systems utilizing technology such as wideband code division multiple access (W-CDMA) will offer a wide array of multimedia services.
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ frequency division multiple access (FDMA) to create communications channels. However, the tremendous increase in the number of users of wireless services and the demand for data and other non-voice services have led to the development of other techniques that can utilize the available spectrum in a more efficient manner. These more advanced techniques include time division multiple access (TDMA), in which communications from multiple users are time-multiplexed on frequency bands in system-defined time xe2x80x9cslots,xe2x80x9d and xe2x80x9cspread spectrumxe2x80x9d or code division multiple access (CDMA) techniques in which channels of a system are defined by modulating data-modulated carrier signals by unique spreading codes, i.e., codes that spread data-modulated carriers over the frequency spectrum in which the communications system operates. The use of unique spreading codes for channels allows several users to effectively share the same bandwidth.
In proposed wideband CDMA systems, such as a W-CDMA system conforming to the UMTS/IMT-2000 specifications, downlink (base station to subscriber terminal) signals for different channels within a cell are transmitted synchronously by the base station using a scrambling code specific to the cell. Typically, orthogonal channelization codes or sequences, also known as spreading codes, are assigned to distinct physical channels transmitted in a cell, thus creating orthogonal downlink signals within the cell. If the communications medium in which the signals are transmitted does not introduce delay spread, this orthogonality may be maintained at the receiving terminal, thus reducing the likelihood of multi-user (inter-user) interference. However, if the communications medium in which the signals are transmitted introduces delay spread, orthogonality may not be maintained at the receiving terminal. This can increase multi-user interference, and may degrade performance.
Performance may be severely degraded in the presence of a so-called xe2x80x9cnear-farxe2x80x9d problem, i.e., when a weak desired signal is received at a receiving station along with a strong interfering signal. In a typical uplink to a base station, this problem may be managed by power control techniques, e.g., by boosting the desired signal such that all signals arrive at the base station at substantially the same power. However, such power control typically is not feasible for a downlink to a subscriber terminal.
It is known that a signal transmitted over a wide band of frequencies generally may produce more multipath signal components than a signal transmitted over a narrower bandwidth. Thus, for example, a channel in a wideband CDMA system generally exhibits a higher degree of dispersiveness than a channel in a narrower bandwidth system such as a system conforming to the IS-95 CDMA standard. Consequently, W-CDMA systems generally have a higher likelihood of multi-user interference than their narrower-bandwidth precursors.
Moreover, proposed W-CDMA systems that allow for the use of variable spreading factors to allow users to achieve varying data rates may be more vulnerable to multi-user interference. For example, proposed W-CDMA systems envision the use of high spreading factors (on the order of 128) for voice channels and the use of lower spreading factors for high-speed data services. If such voice and data services are designed to exhibit comparable link quality, i.e., comparable end-to-end user data reliability, the low-spreading factor signals will generally be transmitted with much higher power than the high spreading factor signals. This power discrepancy may exacerbate multi-user interference in a dispersive medium.
Variable spreading factor schemes which allow for the concurrent use of high and low spreading codes can also exacerbate the xe2x80x9cnear-farxe2x80x9d problem. For example, a user located near the edge of a cell transmitting with a low spreading factor and high power may significantly interfere at a receiving terminal with a high-spreading factor, low-power user positioned nearer the receiving terminal. In addition, a signal with a low spreading factor can degraded by a relatively low power interferer signal, as the lower spreading factor generally renders the desired signal less amenable to interference suppression than a signal using a higher spreading factor.
Interference cancellation techniques have been proposed, but these techniques are generally better suited to application at base stations than at mobile terminals. These conventional techniques typically act on the assumption that the receiver has a priori knowledge of the spreading sequences currently in use in a cell. These conventional interference cancellation techniques also tend to be complex, which can result in significant equipment cost and power consumption that may be disadvantageous in devices such as battery-operated hand-held terminals.
In light of the foregoing, it is an object of the present invention to provide communications apparatus and methods that can reduce multi-user interference in spread-spectrum communications systems.
It is another object of the present invention to provide communications apparatus and methods can reduce multi-user interference in systems using variable spreading factors.
It is another object of the present invention to provide communications apparatus and methods for multi-user interference cancellation that can be implemented in manner that is less complex and power consuming than conventional techniques.
These and other objects, features and advantages are provided according to the present invention by spread spectrum communications systems and methods in which multipath signal components of a received signal are correlated with a set of possible spreading sequences including a desired sequence, preferably using a fast Hadamard transform. The resulting correlations are combined to detect one or more interferer sequences, and interference components associated with the detected interferer sequences are canceled from correlations with the desired sequence to produce interference-canceled correlations. A symbol estimate is generated from the interference-canceled correlations. The interferer sequences may be detected by maximal ratio combining the correlations generated by the fast Hadamard transform and identifying one or more interferer sequences based on whether an energy associated with the interference sequence meets a predetermined criterion such as a threshold value or a nearest neighbor rule. Identification of interferer sequences may be done on a symbol by symbol basis or, in fixed spreading factor systems, interferer sequences may be identified on an intermittent basis, i.e., in selected symbol periods, and reduced-complexity correlations performed for the identified sequences in other symbol periods to generate interference component estimates. Interference cancellation techniques are thereby provided that are relatively less complex and power consuming than conventional techniques.
In particular, according to the present invention, a symbol is transmitted in a communications medium according to a first spreading sequence of a set of spreading sequences that may be used to transmit symbols in the system. A communications signal is received from the communications medium, for example, at a mobile terminal. The received communications signal is resolved into a plurality of signal components, a respective one of which is associated with a respective propagation path. For example, the resolved signal components may be generated by descrambling the received signal according to a cell-specific scrambling code. The resolved plurality of signal components is correlated with the set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with the first spreading sequence. An interference component associated with a second spreading sequence of the set of spreading sequences, identified based on the correlations of the resolved signal components with the set of possible spreading sequences, is cancelled from the correlations of the plurality of resolved signal components with the first spreading sequence, generating a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence. The transmitted symbol is estimated from the set of interference-cancelled correlations.
Preferably, the set of spreading sequences is orthogonal. More preferably, the set of spreading sequences comprises a Hadamard code, and the correlations of the set of possible spreading sequences with the resolved signal components is generated using a fast Hadamard transform.
According to an aspect of the present invention, interference cancellation is achieved by combining correlations for resolved signal components to determine respective energies for respective spreading sequences, and identifying an interferer spreading sequence having an associated energy meeting a predetermined criterion. The combining may be accomplished using, for example, maximal ratio combining techniques, or other combining techniques such as interference rejection combining (IRC). The predetermined criteria may be, for example, a highest energy criterion or a threshold criterion.
According to another aspect of the present invention, symbol and power estimates are generated for an interferer signal(s) transmitted according to the identified sequence(s), and the interference components are determined from the symbol and power estimates. The symbol and power estimates may be generated from the output of the combining process.
According to yet another aspect of the present invention well suited for use in a system using fixed spreading factor codes, interferer sequences may be identified on an intermittent basis and used to generate estimates of interference components during intervening time periods. In other words, an interferer sequence may be detected for a first symbol period of a desired sequence, and used to generate an estimate of a corresponding interference component during a subsequent second symbol period of the desired signal.
An apparatus for recovering a symbol from a communications signal comprises a multipath resolver operative to resolve the communications signal into a plurality of signal components, a respective one of which is associated with a respective propagation path. A spreading sequence correlator is responsive to the multipath resolver and operative to correlate resolved signal components with a set of spreading sequences to generate a respective set of correlations for a respective one of the resolved signal components, a respective one of the sets of correlations including a respective correlation of a resolved signal component with a first spreading sequence. An interference canceler is responsive to the spreading sequence correlator and operative to cancel an interference component associated with an interferer using a second spreading sequence of the set of spreading sequences from the correlations of the plurality of resolved signal components with the first spreading sequence based on the correlations of the plurality of resolved signal components with the second spreading sequence, thus generating a set of interference-cancelled correlations of the resolved signal components with the first spreading sequence. A symbol estimator is responsive to the interference canceler and operative to estimate the transmitted symbol from the set of interference-cancelled correlations of the resolved signal components with the first spreading sequence. Improved spread spectrum communications may thereby be provided.